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CADDIS Volume 2: Sources, Stressors & Responses

energy sources physical habitat hydrology temperature water and sediment quality stormwater runoff wastewater inputs riparian and channel alteration urbanization

The urban stream syndrome

Common effects of urbanization on stream ecosystems have been referred to as the "urban stream syndrome" (Walsh et al. 2005a). Table 2 lists symptoms typically associated with the urban stream syndrome. Symptoms preceded by an arrow have been observed to consistently increase (↑) or decrease (↓) in response to urbanization, while symptoms preceded by a delta (Δ) have been observed to increase, decrease, or remain unchanged with urbanization.

As the urban stream syndrome illustrates, these streams are simultaneously affected by multiple sources, resulting in multiple, co-occurring and interacting stressors. As a result, identifying specific causes of biological impairment in urban streams, or the specific stressors that should be managed to improve condition, is difficult. Some communities are approaching this challenge by managing overall urbanization, rather than the specific stressors associated with it—for example, by establishing total maximum daily loads (TMDLs) for impervious surfaces, rather than individual pollutants.

Many characteristics of urban development affect how the urban stream syndrome is expressed within a given system. These characteristics include (but are not limited to):

Location and distribution of development

Density of development

Type of development and infrastructure
  • residential vs. commercial/transportation
  • stormwater systems
  • wastewater treatment systems

Age of development and infrastructure

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Table 2. Symptoms generally associated with the urban stream syndrome
Stressor Category Symptom
Water / sediment quality ↑ nutrients
↑ toxics
Δ suspended sediment
Temperature ↑ temperature
Hydrology ↑ overland flow frequency
↑ erosive flow frequency
↑ stormflow magnitude
↑ flashiness
↓ lag time to peak flow
Δ baseflow magnitude
Physical habitat ↑ direct channel modification (e.g., channel hardening)
↑ channel width (in non-hardened channels)
Δ pool depth
↑ scour
↓ channel complexity
Δ bedded sediment
Energy sources ↓ organic matter retention
Δ organic matter inputs & standing stocks
Δ algal biomass
Modified from Walsh CJ et al. 2005. The urban stream syndrome: current knowledge and the search for a cure. Journal of the North American Benthological Society 24(3):706-723i.

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